A very simple example for firebase topics

flutter-firebase-topics-example.dart

import 'package:firebase_messaging/firebase_messaging.dart';
import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(
      home: MyApp(),
    ));

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  FirebaseMessaging _firebaseMessaging = new FirebaseMessaging();

  @override
  void initState() {
    //for example your topic name is news
    _firebaseMessaging.subscribeToTopic('news');
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('text'),
      ),
      body: Container(
        padding: EdgeInsets.all(32.0),
        child: Center(
          child: Column(
            children: <Widget>[],
          ),
        ),
      ),
    );
  }
}